Andrew Parker's warning comes just before the Investigatory Powers Bill - or 'snooper's charter' - is about to be published

ISIS terrorists are planning mass casualty attacks in Britain the head of MI5 has warned.

MI5 Director General Andrew Parker said that the terror threat in Britain is at the highest level it’s ever been in the course of his 32 year career.

His remarks come just days before the government’s controversial Investigatory Powers Bill, better known as the Snoopers Charter, is due to be debated in the House of Commons.

The bill, due to be published by parliament on November 10th, will outline the powers of the intelligence services and the police to use surveillance to carry out investigations and gather intelligence in the digital age.

RT reports: Parker praised the work of his intelligence service, but said it would not be able to keep Britain safe unless it had permission to intercept communication data.

The director said Islamic State presents a “three dimensional” threat – at home, overseas and online – with more and more of MI5’s casework related to Syria and the extremist group.

“We are seeing plots against the UK directed by terrorists in Syria; enabled through contacts with terrorists in Syria; and inspired online by ISIL’s sophisticated exploitation of technology,” he said.

“It uses the full range of modern communications tools to spread its message of hate, and to inspire extremists, sometimes as young as their teens, to conduct attacks in whatever way they can.”

Speaking in London on Wednesday at a Lord Mayor’s event, Parker expressed his support for the new security bill, claiming any powers given to the security services must have “strict safeguards” in place.

“We do not seek sweeping new intrusive powers in that legislation, but rather a modern legal framework that reflects the way that technology has moved on, and that allows us to continue to keep the country safe,” he said of the bill.

Parker highlighted the importance of intercepting communication data, but added many forms of communication data are now unreachable be security services as apps and messaging services become more sophisticated.

“It may not yet have reached the high water mark, and despite the successes we have had, we can never be confident of stopping everything,” he said.